If a charged external object such as a human body approaches or touches a microphone, spark discharge occurs between the external object and the microphone. The spark discharge between the external object and the microphone causes noise in audio signals output from the microphone. For example, if a charged human body approaches a microphone, such as a gooseneck microphone, including a sound collector to operate near a mouth, then spark discharge occurs between the human body and the microphone due to a large difference in electrical potential therebetween. Another common problem is the spark discharge between the microphone and a hand of a speaker approaching the microphone for adjusting its position.
The spark discharge occurs when the voltage across the microphone and the human body exceeds the dielectric breakdown voltage of air. In order to eliminate unpleasant noise from output audio signals, the audio signals should be cut off through detection of the static electricity charged in the human body before the occurrence of spark discharge caused by the human body approaching the microphone. Even if some noise occurs in audio signals, a reduction in the audio signal level before the occurrence of spark discharge can minimize adverse effects of the noise.
An exemplary gooseneck microphone that can reduce the noise caused by the contact with a hand is disclosed in Japanese Unexamined Patent Application Publication No. 2012-60490. The gooseneck microphone disclosed in Japanese Unexamined Patent Application Publication No. 2012-60490 includes an external metal cylinder, and an internal cylinder disposed inside the external cylinder and electrically connected to the shielded wire of a microphone cable and the ground pattern of a circuit board. The external cylinder is covered with a cylindrical resin cover. The resin cover has a high volume resistivity. The gooseneck microphone disclosed in Japanese Unexamined Patent Application Publication No. 2012-60490 thus can slow down the charge transfer to reduce the noise caused by the discharge upon the hand contact.
Japanese Patent No. 4508900 discloses a microphone including a means for reducing the noise caused by vibrations of a microphone case to mute the output of audio signals through detection of the vibrations, for example. The means for reducing the vibration noise includes a piezoelectric bimorph element to generate electricity in response to vibrations, a light emitting means to emit light based on the electromotive force of the piezoelectric bimorph element, and a photoconductive element to experience a reduction in resistance after reception of the light from the light emitting means. The microphone disclosed in Japanese Patent No. 4508900 can reduce (mute) the output level of audio signals through a reduction in the resistance of the photoconductive element.
Japanese Patent No. 4560424 discloses a condenser microphone that can reduce the audio output if detecting electromagnetic waves, to reduce or suppress unpleasant noise, for example. In the microphone disclosed in Japanese Patent No. 4560424, a light emitting means emits light in response to detection of high frequency, and a photoconductive element experiences a reduction in resistance after reception of the light, to reduce the audio output.
The microphone disclosed in Japanese Patent No. 4508900 reduces the noise caused by vibrations. The microphone disclosed in Japanese Patent No. 4560424 reduces the noise caused by electromagnetic waves. In other words, neither of the microphones disclosed in Japanese Patent Nos. 4508900 and 4560424 cannot reduce the noise caused by static electricity.
The microphone disclosed in Japanese Unexamined Patent Application Publication No. 2012-60490 reduces the occurrence of noise caused by the discharge upon the contact with a hand. According to Japanese Unexamined Patent Application Publication No. 2012-60490, the noise caused by the discharge is reduced through slowing down the charge transfer upon the hand contact, rather than through the electrical isolation of the route of the discharge upon the hand contact from a circuit for outputting audio signals. Unfortunately, the microphone disclosed in Japanese Unexamined Patent Application Publication No. 2012-60490 cannot completely avoid the occurrence of the noise caused by the discharge.